Thermal spraying, also known as flame spraying, involves the melting or at least heat softening of a heat fusible material such as a metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. In a plasma type of thermal spray gun, a high temperature stream of plasma gas heated by an arc is used to melt and propel powder particles. Other types of thermal spray guns include a combustion spray gun in which powder is entrained and heated in a combustion flame, such as a high velocity, oxygen-fuel (HVOF) gun.
One type of thermal spray powder is formed of chromium carbide and nickel chromium alloy. The carbide does not melt well and would be too brittle alone in a coating, so the alloy, typically nickel with 20% by weight chromium, is incorporated in each powder particle to provide a matrix. Chromium carbide and nickel chromium alloy are selected for high temperature, corrosive and oxidizing environments such as in a gas turbine engine, up to about 815.degree. C.
There are three forms of chromium carbide, Cr.sub.3 C.sub.2, Cr.sub.7 C.sub.3 and Cr.sub.23 C.sub.6 according to a standard phase diagram. The first, Cr.sub.3 C.sub.2, is most wear resistant and stable, melting at 1811.degree. C. The second melts at 1766.degree. C. The third, Cr.sub.23 C.sub.6, is least wear resistant and stable, melting at 1576.degree. C. The first and second form have orthorhombic structure, and the third form is cubic.
Present commercially available powders of chromium carbide with nickel-chromium commonly are produced by blending, or by chemical or mechanical cladding of the alloy onto grains of the carbide, or by mixing, sintering and crushing. Such methods are relatively expensive and effect particles with relatively large grains of carbide. During spraying these grains are exposed to oxidizing conditions which decarborize the carbide and introduce oxides into the coatings. Also the larger grains in coatings can cause scuffing of mating surfaces.
A group of chromium carbide powders were introduced recently by Praxair Surface Technologies, Indianapolis, Ind., according to a brochure "CAT Powders - Introducing A Whole New Breed of CrC-NiCr, Powder Technology" (undated). These are CRC-410 (70CrC-30 NiCr), CRC-425 (60CrC-40 NiCr) and CRC-415 (35CrC-65 NiCr). The present inventors obtained an x-ray diffraction analysis of these powders which showed the carbide to be in the form of Cr.sub.23 C.sub.6, and a chemical analysis which determined a ratio (by weight) of chromium to carbon in the powders to be 22.2 for powders designated CRC-410-1 and CRC-425-1, and 37.6 for CRC-415-1.